Watch this brief video by New Scientist showing how our climate is changing

In the absence of aggressive government policies aimed at curbing greenhouse gas emissions, a number of leading organizations, including the United Nations, the World Bank and others, have begun issuing analyses that regard potentially dangerous temperature elevations as not just a possibility should the status quo prevail, but a near certainty even if things start to change.

[For example, a November 2012 report] by the United Nations Environment Program suggested that greenhouse gas emissions levels are currently around 14 percent above where they need to be by the end of the decade in order to avoid what many analysts believe could be a risky level of planetary warming.

That report comes on the heels of a study . . . by the Geneva-based World Meteorological Organization, which stated that human civilization has pumped roughly 375 billion tonnes, or metric tons, of carbon dioxide into the atmosphere since the dawn of the industrial age, when the extraction and combustion of fossil fuels began in earnest.

“These billions of tonnes of additional carbon dioxide in our atmosphere will remain there for centuries, causing our planet to warm further and impacting on all aspects of life on earth,” said WMO Secretary-General Michel Jarraud, in a statement issued Tuesday. “Future emissions will only compound the situation.”

[Last November] the World Bank issued a report suggesting that the climate could warm a full 4 degrees by the end of the century — less than 90 years from now — even if countries fulfill the modest emissions-reduction pledges they’ve already made.

A 4-degree uptick in temperatures is significantly higher than what has long been deemed the maximum amount — 2 degrees Celsius, or 3.6 degrees Fahrenheit — that average global temperatures could rise while still maintaining a climate similar to that in which human civilization has evolved.

That number, measured against things as they existed before the industrial-scale use of fossil fuels got underway, was not considered absolute. But the best evidence seemed to suggest that keeping the Earth’s average temperature from rising much beyond 2 degrees was a worthy goal, not least because larger increases would raise the odds of many unpleasant things taking place: forbidding sea levels, searing heat waves, grinding droughts and the like.

In subsequent years, some prominent scientists argued that even 2 degrees of warming would be disastrous.

But increasing evidence suggests that such distinctions may no longer matter.

Nearly 30 years after the benchmark was proffered, about half the distance to a 2-degree temperature increase, or about 0.8 degrees, has already been achieved. Further, enough carbon dioxide, the chief planet warming gas that arises when coal, oil and natural gas are burned, is already in the atmosphere to raise future temperatures by another 0.8 degrees, even if all the pollution stopped immediately.

As it is, atmospheric levels of carbon dioxide are at an all-time high and are projected to continue booming. . . .

Should these bleak scenarios prove accurate, the World Bank said on Sunday, a variety of unpleasant end-of-century outcomes would seem all but unavoidable: Increasingly acidic oceans that will fundamentally alter the aquatic food chain; rapidly rising oceans; freshwater scarcity, diminished agricultural yields; and a variety of other impacts — most of them landing particularly hard on the world’s poorest. . . .

[S]some experts remain hopeful — albeit increasingly cautiously — that dramatic action could still forestall the most dire implications of rising temperatures, though they say the window for doing so will not be open for long.

“It is still possible to avoid 2-degree warming, and arguing it is too late could very easily be a self-fulfilling prophecy,” said Michael E. Mann, a climatologist and director of the Earth System Science Center at Pennsylvania State University. “That having been said, the real issue is whether or not we have the political will.”

That sentiment was echoed by Jonathan Koomey, a research fellow at the Steyer-Taylor Center for Energy Policy and Finance at Stanford University, and the co-author of the first comprehensive analysis of the 2-degree limit back in 1989.

“We tend to underestimate the possibility for change,” Koomey said “At certain times, when people perceive a crisis, things can change very, very rapidly.”. . .